The present invention relates to optical filters and, more particularly, to laser radar optical filters using frequency conversion.
Many optical filters used for laser radar are available in the prior art. As understood by those skilled in the art each of these known optical filters has its drawbacks. Some examples of prior art optical filters known in the art include atomic line filters, Faraday anomalous dispersion optical filters, Lyott filters and interference filters.
Atomic line optical filters and Faraday anomalous dispersion optical filters may be treated together because they are based upon quantum effects in gases. The atomic line optical filters are based on absorption effects in various atomic gases. The Faraday anomalous dispersion optical filters are based on dispersion effects in various atomic gases which are subjected to magnetic fields. It is well known in the prior art that cesium is a commonly used gas for both of these types of optical filters.
Unfortunately, these two types of optical filters are not tunable over a broad range of filter frequencies in the manner required for many applications. Additionally, it may be difficult to provide the gaseous cells required for these filters. Furthermore, because these optical filters involve transitions and resonances of atoms within gases, they impose stringent constraints on the corresponding laser transmitters which are used with them.
Lyott filters, which are based on birefringent effects, have their own difficulties. These filters are plagued by materials problems and a limited field of view. However, they do have better tunability than the atomic line filters and the Faraday filters. The other type of optical filters known in the art, interference filters, have a wide bandwidth. Thus they have a lower signal-to-noise ratio and poor transmission which limits efficiency.
The present invention comprises a method and system for overcoming these constraints wherein a wide bandwidth permits relaxation of the constraints on the transmitting system while still permitting a high signal-to-noise ratio and a good field of view. In this system two linear filters are selected to prevent light from passing through both of them. Between the two filters a nonlinear optical device such as a crystal is provided in order to provide sum or difference frequencies which are based upon an input signal which may contain information. The sum or difference frequencies contain the information of the original input signal but are in a different frequency band. This permits the passing of information through the second linear filter which blocks the remainder of the light energy carrying the noise.